Rates of Reactions Teacher Version

 Rate measures how fast or slow a chemical reaction occurs  Who cares about rates of reactions? ◦ Chemical engineers must know how long a reaction takes and how much product is produced ◦ Chemical reactions in fireworks ◦ Objects rusting ◦ Concrete setting

 During a reaction we can measure one of two things: ◦ How quickly the reactants are used up ◦ How quickly new products are produced What happens when we add zinc to sulphuric acid?  Identify the products  Write down the equation for the reaction  What makes this reaction easy to identify a rate?

1)Where is the reaction speed the greatest? 2)How does the reaction speed change as the reaction moves forward? 3)How does the curve change as the reaction speeds up? 4)Describe the graph shape once the reaction is complete (finished).

 The rate changes all through the reaction  The rate is greatest at the start but decreases as the reaction moves forward  The faster the reaction, the steeper the curve  When the reaction is over, the curve goes flat

 Don’t forget the COLLISION THEORY! ◦ Particles can only react if they collide and have enough energy to break the bonds between reactants ◦ More collision means a faster reaction ◦ So … how can we produce more collisions?

 Four factors can affect the rate of a reaction: 1) Concentration 2) Temperature 3) Surface Area 4) Catalyst

 A reaction moves faster when the concentration of a reactant is increased ◦ This increased concentration leads to more particles of the reactant in a given space, increasing the chances of collisions and therefore a faster rate ◦ Ex: Dilute vs. concentrated hydrochloric acid and magnesium

 At a higher temperature, a reaction goes faster  When the temperature increases by 10°, the reaction rate almost doubles!  At an increased temperature, particles have more energy which means they move faster and the chance of collisions increases

Which reaction will proceed at a faster rate?

This reaction produces a fine, yellow precipitate of sulphur

 How to measure the rate: 1) A cross is marked on a piece of paper 2) A beaker containing some sodium thiosulphate solution is put on top of the paper (the cross should still be visible through the beaker) 3) HCl is added quickly to the solution and a timer starts 4) The cross becomes harder to see as the precipitate forms 5) The timer stops at the moment the cross is no longer visible

 Food is kept in the refrigerator to slow down decomposition reactions  What might be some other examples of common temperature/ reaction rates?

 In many reactions, one reactant is a solid. By breaking up the solid (while keeping the mass the same), the rate of reaction can be increased  Ex: hydrochloric acid and marble chips (calcium carbonate) ◦ What is the balanced chemical equation for this reaction?

1) Marble chips and acid are placed in a flask and plugged with cotton wool to prevent liquid from splashing out during the reaction 2) Take the mass of the flask 3) Tip the flask to allow reactants to mix and simultaneously start a timer 4) Note the mass at regular intervals 5) Because the carbon dioxide can escape through the cotton wool, the mass of the flask decreases 6) Experiment repeated twice keeping everything the same EXCEPT the surface area of the marble chips

 A catalyst is a substance that changes the rate of a chemical reaction  The catalyst remains chemically unchanged at the end of the reaction  Catalysts work by giving reacting particles a surface to cling to, increasing collisions and thus reaction rate

 Enzymes are large protein molecules which increase the rate of chemical reactions in the body